1. Field of the Invention
The present invention relates to the art of automatic transmission for a vehicle, and more particularly, to a system for controlling the switching over of speed stages of the automatic transmission.
2. Description of the Prior Art
As an automatic transmission for a vehicle of a relatively high quality type there is known a transmission including two transmission units connected in series, each transmission unit being independently operable to provide a plurality of gear ratios according to switching over of operation of servo actuators incorporated therein, so that a large number of speed stages are available for the total transmission with fine increments of gear ratios. (e.g. Japanese Patent Laying-open Publication No. 57-37140 (1982))
In such a transmission the control of timing is more important in the switching over of operation of the servo actuators for shifting speed stages, because in the upshifting from a certain speed stage to a next higher speed stage a first transmission unit is shifted up while a second transmission unit is shifted down so that the balance of the two changes of gear ratio provides a relatively small increment of the total gear ratio, and therefore, if the shifting down of the second transmission unit precedes the shifting up of the first transmission unit, the transmission is shifted, although temporarily, in the shifting direction opposite to the desired shifting direction.
In view of such requirements it has been proposed in U.S. Pat. No. 4,727,772 corresponding to Japanese Patent Laying-open Publication No. 62-31741 (1987) to control the switching over of operation of the servo actuators in the first and the second transmission unit in coordination with one another. The coordination herein proposed is performed by detecting rotational speed of those rotational members in the first and the second transmission unit which are braked from rotating state toward stoppage or put into rotation from braked state for the switching over of speed stages and by adjusting the operation of the servo actuators in the first and the second transmission unit in accordance with the detected rotational speed of those rotational members so that the respective switching over operations of the first and the second transmission unit are terminated at the same time point.
However, according to the system disclosed in Japanese Patent Publication No. 62-31741 the speed stage shifting performance of the transmission will vary to a large extent, implying that the performance is therefore very unstable, according to small variations of vehicle operating conditions such as engine load and vehicle speed, because the total speed stage shifting performance is influenced by both of the fluctuations in the respective switching over operations of the two transmission units in a manner that a fluctuation in the switching over operation of one transmission unit induces a fluctuation in the switching over operation of the other transmission unit and vice versa so that the instability in the speed shifting of the transmission as a whole is at least doubled by a resonance of instabilities of the two transmission units. Particularly when the two transmission units are switched over in such a manner that one is shifted up thereby to decelerate the engine while the other is shifted down thereby to accelerate the engine, a slight difference in the progressing speed between the two in parallel proceeding switching over operations can cause a great difference in engine load and thereby in engine rotational speed according to which of the switching over of the two transmission units precedes, and therefore the speed change performance of the rotational members upon which the control of operation of the servo actuators is dependent is not constant from one speed change to the next. Therefore, the time required for the same shifting from a certain speed stage to a certain speed stage would vary largely according to slight variations in the vehicle operating conditions. Further, the difference between the time required for upshifting and that for downshifting between certain two speed stages would become too great to allow harmonization of the overall performance of the vehicle.